1. Cross-Reference to Related Application
The invention is related to that described in co-pending Application Ser. No. 144,541 filed May 18, 1971.
2. Field of the Invention
This invention relates to methods and apparatus for depositing transparent, electrically conducting, metal oxide films on to substrates, such as glass, and to articles having such metal oxide films applied thereto. By way of example, the article may be a windscreen, e.g. an aircraft windscreen, on which the film can provide electrical resistance heating means for de-icing or de-misting.
3. Description of Prior Art
Various proposals have been made for reactively sputtering a transparent, electrically conducting, metal oxide film on to the surface of a substrate supported in a vacuum vessel having an atmosphere of oxygen and another gas or gases, from a metal cathode near the substrate surface to be coated. One example of such a process is described in U.S. Patent application Ser. No. 144,541.
Such processes have been successful in producing transparent, electrically conducting, films, of reasonably uniform characteristics on substrates of relatively small lateral dimensions, e.g. 4 cm. in width, but difficulties have been experienced with larger articles. Even though the cathode is enlarged commensurately with the article, so as to cover the whole substrate area and to maintain the direct sputtering path from cathode to substrate at the optimum value (between 2 cm. and 10 cm. depending on the applied potential difference), it is found that the film produced tends to be non-uniform. Variations are found in the thickness and specific electrical resistivity of the film, which result in wide variations in its resistance and diminish or destroy its utility as a resistance heating means. In extreme cases, the film is also found to be less transparent near the middle of the article. Any such reduction in transparency is unacceptable in a windscreen, for example.
The present inventors have deduced that the non-uniformity of the film is due to a reduction in the oxygen concentration in the atmosphere in the working space between the cathode and the substrate, which is believed to be caused by the consumption of the oxygen originally present during the formation of the film, and by the slow rate at which fresh oxygen can diffuse into this space. As the process proceeds, a gradient of oxygen concentration is thus established in the atmosphere in the working space in a plane parallel to the cathode surface, the concentration falling towards the centre of the cathode.
For economy in production, it is desirable to have a high deposition rate and to achieve a minimum specific resistivity (.rho.). However, as the rate of deposition is increased, the rate of consumption of oxygen is also increased and the effect of the gradient in the oxygen concentration will become more severe. Further, as the area of the substrate and cathode is increased so the gradient of oxygen concentration is established over greater distances, the oxygen starvation at the centre of the substrate becomes more pronounced. Consequently it is no longer possible to maintain the desired oxygen concentration necessary to provide a minimum specific electrical resistivity and adequate transparency over the whole area of the substrate to be coated.
The existence of the above-mentioned oxygen gradient has been found to be most detrimental to the deposition of a uniform film. The effect can generally be detected where each lateral dimension, i.e. length and width, of the substrate is substantially greater than the distance between the cathode and substrate, e.g. is greater than 10 cm., and particularly if they are greater than 30 cm.